Alexei Volkov
Plastic is known to shrink when heated, but why does this happen? The phenomenon is known as negative thermal expansion (NTE) and is observed in various materials, including plastics/polymers, certain ceramics, and oxides. When plastic is heated, its polymer chains are no longer locked in a high-strain orientation, and they relax into a low-energy state, resulting in a reduction of the bulk material. This transition to a more stable form is driven by a decrease in Gibbs free energy. Additionally, the internal stress built up during the manufacturing process can be released when heated, causing the plastic to return to its natural, unstretched state. However, the behaviour of different plastic types, such as thermoplastics and thermosets, may vary depending on their structure and the heating process.
| Characteristics | Values |
|---|---|
| Plastic shrinks when heated | True, but only in certain cases |
| Reason | The polymer chains are no longer locked in a high-strain orientation and relax to a low-energy orientation, which curls and bends, shrinking the bulk material |
| Other reasons | Internal stress built up during manufacture, difference in thermal expansion, release of molded-in stress or orientation, increase in bonding between atoms |
| Plastic expansion when cooled | True, this is known as negative thermal expansion (NTE) |
| Plastic expansion when heated | True, but only in certain cases |
| Plastic volume change when heated | The total volume may increase, but the plastic reconfigures from a thin sheet to a thick blob |
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What You'll Learn
Plastic's low melting point
Plastic is known for its low melting point and tendency to soften easily under heat. This is due to its chemical composition, which consists of long polymer chains that are oriented in a particular way during the manufacturing process. When plastic is heated, these polymer chains are disrupted and begin to relax into a lower-energy orientation, which results in the plastic shrinking. This process is not unique to plastic and most materials will expand when heated and contract when cooled.
The melting point of plastic varies depending on its type and composition. For example, Ultem, a semi-transparent, high-strength plastic used in circuit boards and engine components, has a melting point of 219°C. On the other hand, PTFE (polytetrafluoroethylene), commonly known as Teflon, has one of the highest melting points of any thermoplastic at 327°C. This makes it ideal for non-stick cookware coatings, as it can withstand stovetop heat.
PEEK (polyether ether ketone) is another high-performance engineering thermoplastic that can withstand temperatures as high as 310°C for short periods and has a melting point of over 371°C. PEEK is widely used in medical devices, car transmissions, and aircraft exterior parts due to its durability and ease of machining.
While some plastics have relatively low melting points, others with higher melting points may still be susceptible to heat-induced shrinkage. This is because the structural integrity of plastics can be affected not only by their melting point but also by their glass transition temperature (Tg). Above the Tg, plastics become more malleable and can undergo shape changes without actually melting.
The behaviour of plastic when heated also depends on its crystalline structure. Semi-crystalline plastics, such as PE, PET, PTFE, and PP, have a defined melting point and are widely used for the production of fibres, films, and blends. In contrast, amorphous plastics like PS, PC, PSU, and PVC lack a crystalline structure and gradually soften when heated, without a defined melting point.
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Polymer chains relax
The process of plastic shrinking when heated is quite complex and depends on various factors, such as the type of plastic, the manufacturing process, and the presence of internal stress. However, one crucial factor that influences this phenomenon is the behaviour of polymer chains within the plastic.
Polymer chains are integral to the structure of plastic. When plastic is formed, these chains are often oriented in a particular direction, which gives the plastic its toughness and rigidity. This orientation can be achieved through rapid cooling during the manufacturing process, locking the polymer chains in a high-strain arrangement.
When plastic is heated, the behaviour of these polymer chains changes. As the temperature rises, the heat energy is transferred to the molecules within the polymer chains, causing them to become excited and move apart from each other. This movement creates more space between the molecules, leading to an apparent expansion of the plastic. However, this expansion is not permanent.
As the plastic cools down, the molecules in the polymer chains start to move closer together again. If there is no external force or energy holding them apart, they will re-establish bonds and return to a more relaxed, low-energy orientation. This new orientation is typically curled and bent, resulting in a reduction in the overall size of the plastic material—it shrinks.
The shrinkage occurs because the plastic is seeking a more stable and entropically favourable state. The curled and bent conformation of the polymer chains reduces the Gibbs free energy of the system, making it more stable. Additionally, the presence of hydrogen bonding between chain elements may also contribute to the stability of the folded shape.
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Volume increases, surface area decreases
It is a common misconception that plastic shrinks when heated, as the volume of plastic actually increases. However, the surface area of the plastic decreases, giving the appearance of shrinkage. This phenomenon is known as negative thermal expansion (NTE).
When plastic is heated, its molecules gain energy and move apart from one another, causing the plastic to occupy more space. This increase in volume is due to the chains of molecules untangling and creating more open space between them. However, as the plastic cools, the chains of molecules begin to re-entangle and nestle closer together, reducing the surface area of the plastic.
The apparent shrinkage of plastic can be explained by the release of internal stress. During the manufacturing process, plastic is often stretched and moulded into various shapes. This stretching creates internal stress within the plastic, which is released upon heating. Once the plastic cools, it returns to its natural, unstretched state, resulting in a decrease in surface area.
Additionally, the behaviour of plastic when heated depends on its type. Thermoplastic, for example, can be further classified as amorphous or semi-crystalline. Amorphous thermoplastics do not have a sharp melting point, while semi-crystalline thermoplastics do. When heated, semi-crystalline thermoplastics may turn into a liquid state due to their low melting point, and their surface tension causes them to contract and form a sphere.
Furthermore, the method of heating also plays a role in the behaviour of plastic. When heated from one side, the heat distribution is uneven, resulting in one side being hotter than the other. This difference in temperature creates internal stress within the plastic. If the plastic is heated evenly, it softens, and the internal stress causes it to bend or contract.
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Internal stress is released
The phenomenon of plastic shrinking when heated is known as negative thermal expansion (NTE). This occurs when plastic is heated above its glass transition temperature, causing its polymer chains to relax into a low energy orientation. This orientation is characterized by curled and bending polymer chains, resulting in a reduction in the bulk volume of the plastic.
Internal stress plays a significant role in the shrinking process. During the manufacturing process, plastic is often stretched and oriented in a specific direction, creating internal stress within the material. This stress is released when the plastic is heated, allowing it to return to its natural, unstretched state. The heat softens the plastic, making it more pliable, and the internal stress is relieved as the plastic bends or curls.
The release of internal stress can be observed in products like heat-shrink tubing. This tubing is designed with additional mechanical stress, which is released when heat is applied, causing the tubing to shrink and conform tightly to the underlying structure. Similarly, in the case of vacuum-formed plastic products, heating releases the stress that was holding the plastic in its stretched state, allowing it to contract.
It is important to note that the behavior of plastic during heating can vary depending on factors such as the manufacturing process, material composition, and thickness. Different types of plastics, such as thermoplastics and thermosets, may also exhibit distinct responses to heat due to variations in their molecular structures and melting points.
Furthermore, the shrinking of plastic is not solely due to the release of internal stress. The increase in temperature leads to higher rotational energy among the atoms or groups within the polymer chains. This additional energy promotes bonding between the atoms, causing them to move closer together, which contributes to the overall shrinkage of the plastic material.
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Negative thermal expansion
Materials that exhibit NTE behaviour include members of the AM 2O 8 family of materials (where A = Zr or Hf, M = Mo or W) and HfV 2O 7 and ZrV 2O 7. Cubic materials like ZrW 2O 8, HfV 2O 7, and ZrV 2O 7 are especially useful for engineering applications as they exhibit isotropic NTE, meaning the NTE is the same in all three dimensions, making them useful as thermal expansion compensators.
In 2011, Liu et al. demonstrated that the NTE phenomenon originates from the existence of high-pressure, small-volume configurations with higher entropy. They were able to predict both positive and negative thermal expansion in certain materials. Negative thermal expansion is usually observed in non-close-packed systems with directional interactions (e.g. ice, graphene) and complex compounds (e.g. Cu 2O, ZrW 2O 8, beta-quartz).
The concept of NTE is important in the context of plastics. Plastics generally have higher coefficients of linear thermal expansion (CLTE) than metals or ceramics. CLTE measures how much a material expands or contracts with temperature changes. In product design, understanding CLTE helps engineers anticipate dimensional changes that will occur during temperature fluctuations, preventing issues like warping, cracking, or thermal stress.
Plastics have a low melting point and soften easily when heated. Due to surface tension, they tend to pull back into a minimum surface shape, often a ball. This reconfiguration gives the appearance of shrinking, but the total volume of the plastic is actually increasing. The heat disrupts the long polymer chains that are oriented by rapid cooling during the manufacturing process.
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Frequently asked questions
When plastic is heated, its molecules get excited and move apart from one another, creating more space. If no other source of energy is used to lock the molecules in this orientation while cooling, the molecules will re-entangle and nestle together more closely as the plastic cools.
Some plastic products are designed to shrink when heated. This is achieved by adding mechanical stress to the plastic during the manufacturing process, which is then released when the plastic is heated.
Plastic exhibits negative thermal expansion (NTE), meaning it usually expands when cooled. The phenomenon is seen in other materials besides plastics and is almost always related to the geometry of the molecules.
